The manufacture of semiconductor devices typically includes a lithography process. Lithography typically involves various combinations of material deposition, etching, and chemical treatment. A portion of a typical lithography process would proceed as follows. A film (e.g., a metal film) layer is deposited on a substrate. The film layer is typically only a few nanometers (nm) thick. A photoresist layer is then spin-coated on the substrate (i.e., over the film layer). A photoresist (i.e., a positive photoresist) is a photosensitive material that becomes more soluble in aqueous base solvent (developer) upon exposure to light. The photo resist may typically be spun on to the substrate and may include solvents to ensure a uniform coating. Such photoresists may be soft baked after deposition to drive off excess solvents. The photoresist is then selectively exposed to light in specific places. Typically a mask (i.e., a transparent plate having a printed pattern) and a light source (scanner) are used to illuminate the specified portions of the photoresist layer. Then the exposed portion of the photoresist layer (e.g. the portion rendered more soluble in the developer through exposure) is etched. Subsequently the non-exposed portion of the photoresist layer is etched leaving the patterned film layer.
Over the past decade, as the trend toward smaller feature sizes continued, chemically-amplified photoresist technologies have become more prevalent. For example, I-line, with a photoresist wavelength (λ) of 365 nm employed a non-chemically-amplified photoresist. The typical near-UV positive photoresist consists of a polymer (resin) such as novolac and a photoactive dissolution inhibitor (e.g., diazonaphthoquinone (DNQ)). As photoresist technology moved toward deep ultra-violet (DUV) (λ=248 nm, 193 nm) the typical novolac/DNQ photoresist was found to be inadequate. This was due to the inability of such photoresists to become more transparent during exposure (unbleachability) in the DUV region. Chemically-amplified photoresists were developed to address this limitation.
FIG. 1 illustrates the increase in dissolution rate of a non-chemically amplified photoresist in accordance with the prior art. As shown in FIG. 1, the dissolution rate in developer for pure novolac is decreased by the addition of the DNQ. Then upon exposure to the hv light, the dissolution rate increases substantially. This is due to the acid resulting from exposure of the DNQ. In general, for a non-chemically-amplified photoresist scheme, the solubility of the polymer in the developer is greatly increased by acid resulting from exposure of a photo-active compound (PAC).
For chemically amplified photoresists, the mechanism is different. Instead of PAC, Photoacid generator (PAG) is used. The resin (PHOST) in the photoresists are not soluble in developer. Upon exposure to the hv light, the dissolution rate increases substantially. This is due to the acid resulting from exposure of the PAG. The generated acid will deblock the PHOST to form PHS which is soluble in developer. The disadvantage of this approach is that during the post-exposure bake process, the acid produced by the exposure of the photoacid generator (PAG) will diffuse into the film. The diffusion is non-uniform and produces a situation where the polymer lacks sufficient randomness to deblock, which exacerbates the LWR problem for all wavelengths.
FIGS. 2A-2C illustrate the formation of LWR for a chemically-amplified, or non-chemically amplified, resist scheme in accordance with the prior art. As shown in FIG. 2A, a photoresist layer is deposited upon a substrate (i.e., substrate with a film layer deposited thereon). The areas of the photoresist marked with Is indicating the solubility of the photoresist in the developer is inhibited in those areas. As shown in FIG. 2B, portions of the photoresist layer are then exposed to the hv light source increasing the solubility of the photoresist in desired areas. The areas of the photoresist marked with Ps indicating the solubility of the photoresist in the developer is promoted in those areas. Upon development, the exposed portion of the photoresist is removed, as shown in FIG. 2C. However, LWR results due to the acid diffusion. The LWR typically averages more than 10 nm. This was acceptable for feature sizes larger than, approximately, 100 nm, as it amounted to only 10% of the feature size. As the reduction of feature sizes continues, the 10 nm threshold for LWR becomes unacceptable.
In prior art lithography processes, it is not possible to reduce LWR to approximately 1.5 nm, which would be acceptable for 15-16 nm feature sizes.